1. Field of the Invention
The present invention relates to a distance measuring device which emits a laser beam with scanning, receives a light reflected by an object, and detects a distance from the object.
2. Discussion of Background
There have been various proposals for a conventional device for measuring a distance from an object by emitting a laser beam and scanning by a predetermined angle. For example, a conventional device is disclosed in Japanese Unexamined Utility Model Publication No. Hei3-95979 (JP-U-3-5979), and a schematic view of a mechanism for swinging a mirror is shown in FIG. 6. In FIG. 6, numeral 4 designates a mirror for reflecting a laser beam, which is provided on a revolving shaft 5. Numeral 40 designates a permanent magnet having a north pole and a south pole, which are disposed of the circumference of a movable coil 42 wound around a core 41. Numerals 43a and 43b are spiral springs which urge opposing turning effects to each other, wherein one ends of the spiral springs are fixed to vicinities of both ends of the core 41 and the other ends are connected to a driving circuit 44. When an electric current is applied from the driving circuit 44 in the direction of an arrow 45, the mirror is swung in the clockwise direction, and when the current is applied in the reverse direction, the mirror is swung in the counterclockwise direction. It is constructed such that the laser beam can be scanned by this rotation of the mirror. In order to vary a swing angle of the mirror 4 in response to a vehicle speed, the driving circuit 44 applies the current to the movable coil 42.
However, the conventional device has problems that the size of the device was structurally large and miniaturization of the device was difficult because a system of rotating the coil portion by applying electricity thereto while fixing the permanent magnet was used when the mirror was swung; and an accuracy of scanning was not sufficient because an open control was adopted.
The structure of conventional device disclosed in Japanese Unexamined Patent Publication No. Hei6-137867 (JP-A-6-137867) is shown in FIG. 7. In FIG. 7, numeral 1 designates an element for emitting laser beam; and numeral 2 designates a collimating lens for changing the laser beam to a parallel beam. Numeral 3 designates a fixed mirror for reflecting the laser beam by a predetermined angle. Numeral 11 designates a lens for receiving light; and numeral 12 designates an element for receiving light. Numeral 4 designates a swingable mirror for scanning driven by a motor 46. Numerals 47 through 49 constitute a means for detecting a swing angle of the swingable mirror 4, wherein numeral 47 designates a laser diode and numeral 48 designates a condenser. The laser beam emitted from the laser diode 47 is reflected by the backside of the mirror 4 having structure of double-sided mirror. Numeral 49 designates a position sensitive device (hereinbelow referred to as PSD) which detects the position by converting the reflected laser beam to an analog signal.
In this conventional device, the laser beam of the laser beam emitting element 1 was reflected by the fixed mirror and further reflected by the swingable mirror 4 in order to emit to the outside. Meanwhile, the laser beam from the laser diode for detecting scanning position 47 was simultaneously emitted and reflected by the mirror 4 to enter PSD. A feedback control of the motor 46 was performed using such information of position. However, it was necessary to control a deviation of scanning position as well as a deviation of scanning rate to be substantially 0. Therefore, the conventional apparatus has problems that the light emitting element for detecting position, PSD, the lens for condensing the laser beam, the control circuit for controlling these components, high resolution power in these components and so on were necessary; a predetermined area between the swingable mirror 4 and these components was necessary; the cost was high; miniaturization was difficult; and further a method of controlling was complicated, because a controlled variable of feedback control should include two terms of deviation of the scanning position and of the scanning rate.
Thus, in the conventional devices, there were the problems that the mechanism for swinging and the controllability of swinging were complicated as mentioned above.